In 1938, two German chemists, Otto Hahn and Fritz Strassmann, discovered nuclear fission. Fission is the reaction in which the nucleus of an atom splits into two or more smaller nuclei, releasing huge amounts of energy.
Harness this power, physicists said, and you could create a bomb so powerful it could flatten entire cities.
Almost immediately, German scientists commenced work on an atomic bomb project. Backed by a strong German industrial base and military interest, the Uranverein (uranium club) employed some of the world's top nuclear experts.
Although the project was secret, word got out via scientists fleeing persecution in Nazi Germany. Among them was Albert Einstein, who warned US President Franklin D. Roosevelt in 1939.
Anxiety over the development of a Nazi secret weapon rippled around the world.
The US response was The Manhattan Project. Led by J. Robert Oppenheimer, the program began in summer 1942, researching ways to build a fission bomb using the elements uranium and plutonium.
Fear of the rival Nazi project spurred the US government on. With huge financial backing, it took just three years for Oppenheimer and his team to successfully test their first nuclear weapon. The first "live fire" nuclear weapon hit Hiroshimathree weeks later.
Farm Hall recordings
"I don't believe a word of the whole thing," said Werner Heisenberg, then-head of the German nuclear research program, when he heard the news about Hiroshima.
At the time, Heisenberg and the nine other senior nuclear physicists working on the German project were incarcerated in an English estate called Farm Hall. The British secretly recorded the scientists, hoping to discover secrets of the Nazi nuclear projects.
The other German physicists shared his incredulity. Most believed it was a bluff to induce a Japanese surrender. "I didn't think it would be possible for another twenty years," Otto Hahn had said.
Heisenberg and Hahn's reactions show just how far the German program was from developing a nuclear weapon.
"The US completely overestimated the German development of the Uranproject. It wasn't until Farm Hall they understood that," Takuma Melber, a historian at Heidelberg University in Germany, told DW.
Nuclear program scrapped
By the time the The Manhattan Project was up and running, the German nuclear weapons program was already dead. The German researchers knew they would be unable to separate the isotopes necessary for creating an atomic bomb in less than five years. They never achieved a successful chain reaction and had no method of enriching uranium.
The nuclear weapons program was scrapped in July 1942, with the research splitting into nine different institutes around Germany.
"Before 1942 it was a military project, but then it became only a civil project," Melber told DW.
From then on, the goal shifted away from a nuclear weapon to building a nuclear reactor that could sustain nuclear fission on a smaller scale. Heisenberg moved his research to a cave laboratory under a castle in Haigerloch, Germany, where he and his team built an experimental nuclear reactor comprised of uranium cubes dangling from wire and submerged in a tank of heavy water.
This experiment was the furthest the German nuclear program progressed, but the reactor never worked — there wasn't enough uranium present in the reactor's core to achieve a chain reaction.
But they were close. Scientists now believe that if the reactors had contained 50% more uranium, Heisenberg could have created the first nuclear reactor.
Werner Heisenberg's experimental nuclear reactor paved the way for the development of nuclear power stations in the mid-20th century Deutsche Welle
German disorganization and persecution
With a head start and brilliant scientists working on the project, why did Germany fail to develop its nuclear program?
For one, Germany was bleeding scientists. Many Jewish and Polish scientists like Lise Meitner, a Jewish physicist who played an instrumental role in Hahn and Strassmann's discovery of nuclear fission, fled persecution. A number of these refugees fled to the UK and US, where they worked on the Manhattan Project.
Other scientists were drafted into the German army.
Wartime pressure in Germany also rendered scarce some of the resources necessary for the research, like sufficient amounts of enriched uranium, said Melber. Water, which is needed to cool nuclear reactors, was also in short supply.
"Heavy water production was underway in Nazi-occupied Norway, but Allied and Norwegian forces attacked these facilities," said Melber.
But ultimately it was the lack of political support that halted progress.
"Hitler had difficulties understanding the project" and cut support of it in 1942, Melber said. Without this backing, the nuclear program had very few resources to draw on, especially compared to the US Manhattan Project, which employed 500,000 people, about 1% of the US workforce, and cost the US government around $2 billion (today around $24 billion, or €22 billion).
By comparison, the Uranverein and subsequent programs involved fewer than a thousand scientists and were budgeted at 8 million reichsmarks, equivalent to about $24 million dollars today.
A congress of international scientists in 1927. Among them are Werner Heisenberg, who led the German nuclear project, and Niels Bohr, who worked on the rival Manhattan Project Deutsche Welle
Make reactors, not bombs
The Farm Hall tapes also provide another reason for the German failure — the scientists themselves were morally opposed to the atomic bomb and secretly sabotaged the effort.
One of the scientists, Carl Friedrich von Weizsaecker, said, "I believe that the reason we didn't do it was because all the physicists didn't want to do it, on principle. If we had all wanted Germany to win the war, we would have succeeded."
Heisenberg himself was opposed to the idea of a nuclear bomb, saying "at the bottom of my heart I was really glad it was to be an engine and not a bomb."
The German scientists at Farm Hall went on to hope that "history will record… Americans and the English developed this ghastly weapon of war… and that the Germans produced a workable engine."
Eighty years on, the irony is that the modern German state hosts US nuclear weapons, believing them vital for Germany's security, but is vehemently opposed to nuclear power.